I guess the best one would be the officially supported one XFMG because it has the most third-party plug-ins and of course is supported with your forum subscription .
But I used to also own XenGallery and really miss it. It really did just about everything the official gallery did except he didn't have as many third-party add ins. I just preferred the way it looks with infinite scrolling etc. it really has The most modern feel in my opinion.